Makes quenching metal sproede

Quenching and tempering is a process for the heat treatment of metals in which hardening is combined with subsequent tempering. As a rule, the aim of hardening is to create a hard structure consisting of martensite, bainite or a mixture of these two. The tempering process causes a thermally induced structure formation or structure change of the material.

The hardening

Only hardenable materials can be tempered. This includes both metals such as steel and non-ferrous metals such as titanium alloys. The hardenability of a material depends on whether it can form a martensite or bainite structure. In addition, the grain size of the structure also influences the temperature-dependent transformation processes and thus the heat treatment of the material.

For classic tempering of steel, the material must have a carbon content of 0.2 to 0.3%. In the quality of so-called quenched and tempered steel for mechanical engineering, the steel usually has a carbon content of 0.35 to 0.6%. Steels with other properties are more suitable for so-called surface hardening, as they harden more poorly. The thickness of the surface layer can be adjusted by choosing the right alloying elements.

Quenching and tempering process - hardening, quenching & tempering

To harden a workpiece, heat it up quickly at> 4 K / min above the austenitizing temperature. Avoid heating too quickly to prevent warping and cracking.

The deterrent

Quenching is the rapid cooling of a heated workpiece using a quenching agent. The most common deterrents used are water, oil, or air. The quenchant affects the quench rate vK and thus also the structure created in the material. The maximum quench rate that can be achieved with mineral oils as the quenchant vK lies in a temperature range of 150-200 ° C / s. If you use water as a deterrent, you will achieve a quenching speed three times faster than with mineral oil.

In the case of hypoeutectoid steels, quenching should take place in a temperature range of 30 to 50 ° C above the temperature AC3 defined in the iron-carbon diagram. In the case of hypereutectoid steels, the material should have reached a temperature just above AC1, the temperature defined in the iron-carbon diagram, before quenching.

The holding time tH depends on the thickness s of the workpiece. You can use the following rule of thumb to calculate the holding time tH estimate.

The carbon is dissolved in the austenite. In order to completely dissolve the carbides, you need an increased austenitizing temperature. However, since martensite is formed as a result, the material becomes brittle accordingly, which is why an increased austenitizing temperature is not recommended under these conditions. However, if you fall below the austenitizing temperature, there is a possibility that ferrite nuclei that are too soft will develop in the hard martensite structure. This phenomenon is also known as soft speckling, which makes processing the material more difficult and also reduces the service life of the tools used.

The starting

For complete tempering, it is best to have a tempering step at approx. 150 ° C immediately after quenching. This process transforms the brittle tetragonal martensite or needle martensite that formed during the hardening process into a cubic martensite structure. At the same time, the material separates fine carbides. In addition, the precipitation of even finer carbides makes it more difficult, for example, for dislocations that result from high loads to slide off. This makes crack formation and crack continuation more difficult and at the same time increases the toughness and hardness to the secondary hardness maximum.

Colors of the tempering stages of steel

After the transformation into a martensite structure, the volume of the material is reduced, which relaxes the grain lattice and eliminates the so-called glass hardness of the material. If you treat the material in further tempering stages with higher temperatures in the range of 200 to 350 ° C, this process will continue. In addition, diffusion processes simultaneously decompose remaining austenite and convert it into martensite. These process steps cause a further increase in the hardness of the material.

If the steels are highly alloyed, iron carbide is converted into harder and more stable special carbides in an additional tempering stage at temperatures of more than 500 ° C. These special carbides consist of carbide-forming alloying elements such as Cr, Mo, V and W.

An overview of the changes in the material properties resulting from tempering can be found in a material-specific tempering diagram.